1. Field of the Invention
The present invention relates to a communication control apparatus, a portable communication apparatus and a communication control method, all for performing communications via a communication satellite and a ground radio base station, and more particularly to a fitted-on type miniature communication control system such as a wrist watch type system.
2. Description of the Related Art
Mobile radiotelephone systems like an automobile telephone and a cellular telephone are nowadays utilized widely. In recent years, new mobile radiotelephone systems such as a PHS (Personal Handy-phone System) in Japan and a GSM (Global System for Mobile Communication) in Europe have been developed. Now the miniaturization of the mobile radiotelephone systems and an improvement in their performances are being carried out in accordance with the digitization of communication/call data. As a result, a small-sized communication terminal having a high performance can be used for sending and receiving data to and from a facsimile and a personal computer.
A system utilizing a communication satellite (geostationary satellite), which moves in a stationary orbit, is mainly used as a satellite communication system. In a satellite communication system utilizing a geostationary satellite, a communication terminal requires a large-sized parabola antenna. A worldwide mobile communication system employing, in place of a geostationary satellite, a plurality of non-geostationary satellites which move in orbits lower in height than the orbit of the geostationary satellite, is being planned now. Among others, considerations and experiments have already been initiated for plans such as the "Iridium Project", "Odyssey", "Global Star" and "Inmarsat-P" in order to bring them into practice.
In the mobile communication system utilizing non-geostationary satellites, one communication terminal outputs radio waves of the same degree as those for a current portable telephone and performs communications with another communication terminal via a communication satellite.
In recent years, a GPS (Global Positioning System) which measures a current position through the use of a GPS satellite has been generally utilized. A navigation apparatus which is mounted in a vehicle is mainly adopted in the GPS. Of late, however, a portable GPS reception apparatus has also been employed in accordance with the miniaturization of a GPS reception unit which receives radio waves from a GPS satellite. In Europe and the United States, a DGPS (Differential Global Positioning System) in which a fixed reception station accurately receives radio waves from a GPS satellite and correction data is created based on the received radio waves, is also utilized in order to improve the measurement accuracy in the GPS.
The mobile radiotelephone systems such as a cellular telephone and the PHS are basically those for achieving a call between communication terminals through a relay radio base station. According to those systems, however, it is also possible to realize a long-distance call between communication terminals and to establish a connection between a communication terminal and a public line network. According to the mobile radiotelephone systems, a service area is divided into radio zones known as intermediate-distance area cells or short-distance area microcells. Radio base stations are provided in the individual radio zones and are in charge of them. The radio base stations are connected to each other via a private line network or a public line network, due to which a call can be made between communication terminals which differ in radio zone and which are far distant from each other, as well as between a communication terminal and a subscriber's telephone connected to the public line network.
In principle, the mobile radiotelephone systems can realize a call when a communication terminal is present in a radio zone where the communication terminal can receive radio waves from a radio base station. In order to cover all areas as the radio zones, the mobile radiotelephone systems have to be provided with a large number of radio base stations.
Depending on country and system, the mobile radiotelephone systems differ from each other in radio frequency, connection method, communication protocol and standards for the communication terminals. These differences make mutual connection and data exchange impossible, which results in that a communication terminal, which is adopted in one mobile radiotelephone system, cannot be used in another mobile radiotelephone system.
In the mobile communication systems such as the "Iridium Project", "Odyssey", "Global Star", "Inmarsat-P", etc. which are being planned now, communications are performed using non-geostationary satellites. In general, a geostationary satellite travels along a stationary orbit whose height is 36,000 km. Meanwhile, a non-geostationary satellite travels along a low earth orbit or a medium earth orbit. The height of the low earth orbit is approx. 500 km to several thousand kilometers, and is lower than that of the orbit of the geostationary satellite. The height of the medium earth orbit is approx. 10,000 km, and is located inside the Van Allen belts. In the mobile communication systems utilizing non-geostationary satellites, since the height of the orbit of a communication satellite is low, the radio wave propagation delay time is short and the propagation loss of radio waves is small. Those permit the size and weight of a communication unit, etc., which are mounted in the communication terminal or the communication satellite, to be reduced.
The mobile communication systems utilizing non-geostationary satellites have an advantage in that a call area, which is considerably wider than a radio zone (call area) where radio waves can be received from a radio base station, can be attained. However, the above-mentioned mobile communication systems have the following problem:
A non-geostationary satellite moves relative to the earth at high speed, and accordingly the period of time over which the communication satellite stays in a communicable area, where it can communicate with a communication terminal, is short. In the case where the non-geostationary satellite moves in an orbit whose height is 1,000 km, the period of time over which the satellite stays in the communicable area, which begins at a specific point on the ground, is only twelve minutes. In order to enable a call to be made continuously from the specific point on the ground, a large number of non-geostationary satellites are required, and those satellites have to be successively located in communicable areas where they can communicate with the communication terminal. Since the speed at which the non-geostationary satellites move relative to the earth is high, a frequency shift due to the Doppler effect is considerable.
The antennas and the bodies of the communication terminals which are employed in the mobile communication systems utilizing the non-geostationary satellites can be made smaller in size than those employed in a system employing a geostationary satellite, but have larger sizes than those employed in the mobile radiotelephone systems. In the mobile communication systems utilizing the non-geostationary satellites, the communication fees are more expensive than those in the mobile radiotelephone systems, because the costs of launching and controlling a large number of communication satellites are huge.
In order to bring the communication fees down close to the fees charged for use of the mobile radiotelephone systems, it has been considered to employ, in the mobile communication systems utilizing the non-geostationary satellites, dual-mode communication terminals which contain communication circuits for the mobile radiotelephone systems so that not only the mobile communication systems but also the mobile radiotelephone systems can be utilized. According to the mobile communication systems utilizing the non-geostationary satellites and employing the dual-mode communication terminals, when the communication terminals can be connected to radio telephone base stations, the mobile radiotelephone systems, not the mobile communication systems, are utilized in order to reduce the total fees.
In the mobile radiotelephone systems such as the PHS in which a service area is divided into small radio zones, a communication terminal regularly receives an ID code from a radio base station, and identifies the radio base station which is in charge of a radio zone. When the communication terminal receives another different ID code, the communication terminal discriminates that it has moved to another radio zone, and sends a control signal to another radio base station which is in charge of the radio zone to which the communication terminal has moved. The radio base station receives the control signal sent from the communication terminal, and registers information about the communication terminal in the database of a network control apparatus which controls the radio base station. When calling up the communication terminal, the network control apparatus sends out a call signal to the communication terminal through the radio base station in accordance with the communication terminal information registered in the database, and establishes a line connection. In order to connect a line to the communication terminal, the network control apparatus needs to constantly check in which radio zone the communication terminal is located. Meanwhile, the communication terminal needs to regularly receive an ID code sent from the radio base station, which results in an increase in the power consumption.
In the case of a mobile communication system employing a geostationary satellite which moves in an orbit whose height is 36,000 km, the communicable area is wide, since the altitude of the communication satellite is high. In the mobile communication system utilizing a geostationary satellite, the communication satellite need not register or control the communication terminal, because the communicable area is fixed. On the other hand, in the mobile communication systems utilizing the non-geostationary satellites which move in the orbits whose heights are lower than that of a geostationary satellite, the communicable area changes from one to another in accordance with the movement of the communication satellite, and therefore the communication satellite needs to register and control the communication terminal.
In the mobile communication systems utilizing the non-geostationary satellites, unlike in the case of the mobile radiotelephone systems, the communication satellite is always moving relative to the earth, and the speed of its movement is considerably high. The network control apparatus has to frequently register the communication terminal and the communication satellite corresponding to the communication terminal in the database. Meanwhile, the communication terminal has to frequently receive an ID signal sent from the communication satellite. Consequently, according to the mobile communication systems utilizing the non-geostationary satellites, the important communication line and the electric power of the communication terminal are consumed due to such control procedures other than communications.
In order to eliminate the need to perform such control procedures other than communications in the mobile communication systems utilizing the non-geostationary satellites, it has been proposed to measure the position of the communication terminal, determine from the position of the communication satellite the communication satellite which becomes possible to communicate with the communication terminal, and perform communications via the communication satellite. Two methods for measuring the position of the communication terminal are available. According to one method, the communication satellite measures the position of the communication terminal. According to the other method, the communication terminal measures its position. However, those two methods have the following problems:
According to one method in which the communication satellite measures the position of the communication terminal, as well as in the case of an Emergency Position Indicating radio Beacon, the communication terminal has to constantly or frequently send position measurement data to the communication satellite, under which condition the communication terminal consumes a large amount of power. In the case where the number of system subscribers increases, the communication satellite is required to process position measurement data sent from a huge number of communication terminals, which entails the need to provide the satellite with a large-scale processing unit.
According to the other method in which the communication terminal measures its position, three or four communication satellites are located in the sky and send position measurement data in the same principle as that of the GPS. In this method, the number of communication satellites, the determination and arrangement of the orbit are limited. The communication satellites send the position measurement data to the communication terminal, and relay, to an earth station, position information sent from the communication terminal. The communication terminal thus requires a circuit for receiving the position measurement data, and consumes a large amount of power in order to receive the position measurement data.